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Sonicaid Team Operator’s Manual
72
The measurement of short-term variation, in the absence of episodes of high
variation, is independent of basal fetal heart rate, and correlates with the
development of metabolic acidaemia and intrauterine death as follows:
STV % likelihood of metabolic
(ms) acidaemia or intrauterine death
> 4 0
3.5-4.0 8
3.0-3.5 29
2.5-3.0 33
< 2.5 72
STV has been shown to be an excellent indicator of fetal well-being.
Episodes of high and low variation
An episode of high variation is defined as a section of the trace where the one-
minute peak-to-peak variation is above a given threshold for 5 out of 6 consecutive
minutes. An episode of low variation is defined as a section of the trace where the
one-minute peak-to-peak variation is below a given threshold for 5 out of 6 consec-
utive minutes.
The threshold for high is defined by a pulse interval of 32 ms, and the threshold for
low by a pulse interval of 30 ms. The variability for each episode is expressed in beats
per minute, and is independent of the basal heart rate.
High variation occurs when the fetus is in an active sleep phase, and low variation
when it is in quiet sleep. As the fetus matures, episodes of high variation increase,
and episodes of low variation decrease. One of the Dawes/Redman criteria is for
there to be an episode of high variation where the peak-to-peak variation for the
episode is greater than the first centile, when corrected for gestational age.